It is a great challenge in the orthopedic and periodontal surgical fields to find systems for treating patients with skeletal disorders and deformations, including the repairing of large bone defects originating from trauma, excision of tumours and congenital malformations, reconstructing bone stocks worn off by an implanted endoprothesis in revision operations and healing delayed or non-united fractures.
Autogeneic cancellous bone grafts harvested from human bone have been the most reliable and effective alternative to bone substitute so far. However, restricted availability of sources, suffering caused by explanting surgery and the risk of transmission of human immunodeficiency virus and of other complications limit its extensive clinical use. Raised incidences of stress fracture, nonunion and failure of incorporation of cancellous bone grafts have also been reported as the defect sizes extends. The synthesized biomaterials which have been commercialized can only be used as filling material or supporting scaffold without biological activity in initiating bone regeneration. A desirable bone substitute such as investigators and clinicians have sought might be a reconstitution of synthetic material possessing chemical composition, geometrical architecture, mechanical integrity and biocompatibility similar to those of the bulk of living bone, with biological growth factors able to induce or improve bone regeneration. Such a substitute can hopefully replace autografts and be extensively applied in all contexts related to hard tissue transplantation in clinical medicine. Commercially available synthesized biomaterials have been developed and can be used as filling material or inlay as well as onlay support. Unfortunately, these materials lack the biological activity in initiating bone regeneration. Synthetic carriers prepared from such materials as polylactic acids and hyaluronic acids are described e.g. in the patent U.S. Pat. No. 5,366,508. Bone morphogenetic protein (BMP) is an important factor in osteogenic devices and participates actively in the implantation process.
BMP has been the target for a tremendous amount of research, since the significance of BMP in skeletal biology was first recognized and the methods for its extraction and purification were published by Urist—one of the pioneers in the field of BMP research. BMP has also occasionally been used for treating patients with delayed union of fractures, but it has not yet been used in great scale for treating patients.
Important milestones in the history of BMP are the discovery of the effectiveness of BMP in induction of new bone formation, the development of isolation methods for partially purified naturally occurring BMP from animal bone matrix and the isolation of different cDNAs encoding BMPs, which enables the preparation of BMP with recombinant DNA techniques.
The development of recombinant DNA techniques makes it possible to produce recombinant human bone morphogenetic proteins (rhBMPs). However, the rhBMPs have not yet been approved for use in clinical treatments. Preliminary experiments have indicated that rhBMP has a biological activity and probably may have some immunological advantages compared with native BMPs. However, the biological activity of rhBMP is only one tenth or less of the biological activity of purified or partially purified native BMP (Bessho, K., et al., Personal Communications; Protein, (In press) 1966).
The preliminary results have led to the development of a multitude of biological delivery systems for BMP in vivo. Even if the experiments with highly purified naturally occurring BMP or recombinant BMP given without carriers have given positive results, the BMP without a carrier rapidly dissolves and diffuses in the body fluids after implantation and soon thereafter expression of osteoinductivity is impeded. Thus, an appropriate functional carrier of BMP is needed to potentate and modulate the activitity of BMP. However, so far the results with BMP isolated with conventional methods or rBMP combined with conventional carriers have not been as encouraging as expected.
The carrier or delivery system of the osteoinductive BMP in implants has a great effect on the biological activity of BMP. The carrier protects BMP from rapid outward diffusion and endogenous proteinization. It retains a persistent concentration gradient of BMP coinciding with differentiation of BMP target cells and osteogenesis. Further, it provides a scaffold for attachment of BMP responsive cells.
Delivery systems for BMP has not only been discussed in a multitude of publication, they have also been extensively disclosed in several patents and patent applications. For example, the patent U.S. Pat. No. 5,443,531 discloses a delivery system in which BMP is adsorbed on a hydroxyapatite carrier in a chromatography column.
In order to improve the properties of the delivery systems for administering BMP in clinical applications, collagens, especially type IV collagen, have been used to impregnate carriers and improve biological activity. In the patents U.S. Pat. No. 4,975,527 and U.S. Pat. No. 4,394,370 collagen based carriers for BMP are disclosed. The bone inductive potential and dose-dependent response of bovine BMP combined with a type IV collagen carrier is discussed in Gao et al., Ann. Chir. Gynec. 82: 77–84, 1993.
A desirable bone substitute would be a material lacking the risks related to human derived autografts, possessing the properties of a synthetic material, i. e. having a chemical composition and a geometrical architecture and a mechanical integrity and strength similar to the bulk of living bone autografts and at the same time having the biological activity including growth factors, which are able to induce or improve bone regeneration.
Gao et al., (Biomaterials 16, 1175–1179, 1995) have published an article about the microscopic evaluation of bone implant contact between hydroxyapatite, bioactive glass and tricalcium phosphate implanted in sheep diaphyseal defects.
Promising results have been obtained with so called bioceramics, such as biocorals originating from the skeleton of corals. Biocoral have been extensively used as carriers. The use of calcium carbonate originating from coral skeleton as bioresorbable bone material has been described e.g. in the patent U.S. Pat. No. 5,433,751 and the patent publication WO 93/02181.
The use of the biocoral carriers in combination with certain growth factors, such as TGF, has been disclosed in the patent publication WO 94/26322. The use of the growth factor TGF in combination with BMP is also disclosed in the patent U.S. Pat. No. 5,393,739.
Even if a multitude of different delivery systems for BMP have been suggested and different systems have been experimentally tested (Lindholm & Gao, Ann. Chir. Gynaecol. 82, 3–12, 1993) no fully satisfying combination of BMP and carrier has so far been discovered for clinical treatment of human patients.
It has also been shown that BMPs isolated by the conventional methods in addition to decreased induction of bone formation cause immunogenic and inflammatory reactions.
Thus, there is a great need to develop novel alternative, osteogenic devices for improved delivery of BMP to obtain an improved osteoinductive effect. At the same time it is important to obtain an osteogenic device with decreased immunogenic properties.
The main objective of the present invention is to provide an osteogenic device which has improved inductive activity in bone formation and with less immunogenic and inflammatory reactions, which device is useful in ectopic bone induction and healing of segmental bone defects in vertebrates.
Another objective of the present invention is to provide a novel osteogenic device with improved osteoinductive properties for improved bioactive delivery of BMP, especially modified BMP.
A further objective of the present invention is to improve the bioactive delivery system for BMP by increasing the biological potential of the delivery system by impregnating the carrier, especially a bioceramic carrier with collagen.
The objective of the present invention is to provide a new osteogenic device with improved resorbability.
It is further an objective to provide a modified BMP complex with improved storability.